The subject invention relates generally to variable capacitance devices and particularly to such devices employing field effect technology which are useful for monolithic microwave integrated circuits (MMICs).
Present day electronic applications often require the utilization of variable capacitive elements which are responsive to control signals to vary the amount of capacitance provided thereby. For example, such variable capacitors are useful for tuning the bandpass characteristics of amplifiers, changing the frequency of Voltage Controlled Oscillators and for adjusting the frequency responses of variable filters.
Prior art, electronically variable capacitive devices generally utilize varactor diodes. These two-terminal devices require a separate network to isolate the direct current (DC) supply from the radio frequency (RF) signal circuitry. Such networks pass DC from the DC supply but block RF from the DC supply and pass RF from the RF supply but block DC from the RF supply. An isolation network usually includes at least a large valued inductor and sometimes a capacitor. Particularly when MMIC circuitry is used, such isolation circuitry is usually prohibitively large and expensive.
A MESFET having the drain and source thereof interconnected is also sometimes utilized particularly in some digital MIMIC applications to provide a variable capacitor. Bias or capacitance control current flows through the source-to-drain channel. Such MESFETs often require an undesirably large amount of current for a given capacitance. These currents tend to undesirably heat up the circuitry and increase the demands on the power supply, for instance. Also, high current MESFETs provide relatively small resistances and thereby provide poor isolation. Thus, these MESFETs also require separate DC/RF isolation components.